1. Field of Invention
The present invention relates to a drive method for an electrophoretic display device and to an electrophoretic display device.
2. Description of Related Art
Electrophoresis, a technique in which Coulomb's force is used to cause electrophoretic particles to migrate by applying an electric field to a suspension fluid having electrophoretic particles dispersed in a fluid, and electrophoretic display devices that operate by means of electrophoresis, are known from the literature. There is, however, still room for improvement with respect to electrophoretic display devices.
A common drive method used in liquid crystal displays and other types of display devices is to vary the potential of the pixel electrode while also changing the potential of the common electrode (a method also referred to herein as “variable common electrode drive”). Japanese Unexamined Patent Appl. Pub. JP-A-S52-70791 is directed a variable common electrode drive method for an electrophoretic display device.
This drive method controls the potential of the pixel electrode and the common electrode to either of two values, that is, either a high potential or a low potential. This enables lowering the voltage of the drive potential, simplifying the circuit design, and low cost manufacture. When thin film transistors (TFT) are used in the drive circuit, lowering the drive potential as described above can also improve TFT reliability.
Problems with this variable common electrode drive method that remain to be solved are described below.
If a pulse signal with a 200 ms pulse width is applied to the common electrode to change the common electrode potential, flicker results and the flicker can induce user stress. More specifically, when the display is driven by varying the common electrode potential, color changes are sharp when drive starts and flicker occurs easily.
This flicker can be prevented by inputting a pulse signal with a short pulse width such as 20 ms for variable common electrode drive. However, because the pulse width for which voltage is applied is short, the particles cannot be moved enough to saturate the display, and only relatively low reflectance can be achieved.
Sufficient reflectance also cannot be achieved with a single write operation because pulse signals with a short pulse width are used. The reflectance of pixels that are redrawn from white to white and the reflectance of pixels that are redrawn from black to white therefore differs in an electrophoretic display device that uses white and black electrophoretic particles, resulting in an uneven display, ghosts, and other problems.